Aluminum is the most abundant metal found on earth, the oxides and silicates of which comprise about 8% of the earth's crust. A total of about 29 million tons of aluminum is needed to meet worldwide demand each year. About 22 million tons of this total is new aluminum and 7 million tons is from aluminum scrap that is recycled for reuse. Thus, about 25% of the total amount of aluminum used worldwide is recycled. Of the 22 million tons of new aluminum produced every year only 50% is ever recycled.
New aluminum is made from bauxite by way of the Hall-Heroult process that involves electrolysis of alumina (Al2O3) to aluminum metal. The large electrolysis operations that produce aluminum metal typically run continuously 24 hours a day, seven days a week. A modern smelter typically costs on the order of $1.6 billion. It has been estimated that 1% of the energy generated in the United States and 3% of the world's energy is used in making aluminum metal.
Although the electrical voltage used in alumina ore smelters is typically only about 5.15 volts, the amount of current necessary is in the range of 100,000 to 150,000 amperes. In addition, the electrolytic smelting process in carried out at a temperature of about 900° C. The majority of aluminum produced has a purity of 99.7%, however, super pure aluminum having a purity of 99.99% is required for special applications wherein high ductility or conductivity is required. Although these purity differences appear marginal, the difference in properties between the two grades is significant.
In addition to the high energy cost to produce aluminum, the electrolyte used in the Hall-Heroult process, cryolite (sodium aluminum fluoride, Na3AlF6), is an uncommon mineral of very limited natural distribution; it is only found in large quantities on the west coast of Greenland. The cost of obtaining aluminum from the Hall-Heroult process is therefore increased because of the need to synthesize large quantities of cryolite.
The production of secondary aluminum from alloy requires only 5-20% of the energy that is need for the production of primary aluminum from bauxite. The conventional method for recycling either aluminum metal scrap, for example, shavings, or processed aluminum, for example, aluminum cans, involves sizing the scrap followed by compacting. However, the conventional process requires melting aluminum at high temperatures in addition to degreasing and other cleaning steps. High temperature processes result in the loss of aluminum metal from burning.
However, conventional aluminum production methods, as well as recycling processes, produce a large quantity of aluminum dross that is typically skimmed off the top of the aluminum melt and discarded. State of the art dross recycling processes select only the largest pieces of material for processing and therefore recover only from 3-10% of the aluminum. However, these dross recycling processes also involve a large consumption of energy per pound of purified aluminum obtained. In addition, conventional smelting process which recover aluminum metal from dross, also produce dross having a substantial amount, but a lower concentration, of aluminum metal.
There is therefore a long felt need for a process that is suitable for obtaining aluminum from the dross formed during both the primary smelting process, as well as during recycling processes.